1. Field of the Invention
The present invention relates generally to programmable integrated circuit memory devices. More particularly, the present invention relates to a programmable read-only memory (PROM) device having separately programmable output modes.
2. Description of the Related Art
Currently, there are available several types of integrated circuits which provide electronic "memory," i.e., the ability to store digital information. Depending upon the logic function to be implemented, each has certain advantages and disadvantages from the system designers point of view.
One type of integrated circuit in which a fixed set of binary information can be stored is the programmable read-only memory (PROM) device. Basically, the PROM device internally includes a decoder and an addressable array of gates implementing logical functions in a two-level implementation in sum-of-terms form.
For example, the decoder generally comprises a fixed AND gate array by which input variables are decoded and output as the terms of a function. The addressable array comprises OR gates having special internal links that can be fused or severed to form the required circuit paths that "program" the device according to the specifications of the user. In other words, the connections between the outputs of the decoder and the inputs of the OR gates can be specified to implement a certain logic program.
While the use of PROM devices for implementing logic functions has certain advantages over other programmable integrated circuit memory devices (such as speed of operation and ability to decode all input combinations), a major drawback has been that the type of outputs is limited to either combinatorial, as shown in FIG. 1a, or registered, as shown in FIG. 1b, modes only.
FIG. 2a depicts a typical, well-known combinatorial logic-type structure 1. The state of the fuse link 3 will control the gate 5 to invert or not to invert a signal on input 7. The output 9 is commonly known as a "nonstored" or "nonregistered" signal.
FIG. 2b depicts a typical, well-known registered-type structure 11. A data storage register 13 has a data input D coupled via line 7' to one of the input signals, and an output Q coupled via line 9' to one of the single outputs of the circuit or chip. The output as shown in FIG. 1b is commonly known as a "stored" or "registered" signal.
Combinatorial logic output PROM devices are useful when there is a large number of possible input combinations or product term outputs, whereas registered logic output PROM devices are useful for sequential logic, viz., where there is a large number of variables involved in the logic function. For example, a registered PROM device can function as a state machine in which the next output is a function of the present state (stored in the register) and the inputs.
Current generation PROM integrated circuits offer dedicated combinatorial output or registered output structures only. The output structure is fixed and common for all of the output terminals. As a result, system designers must stack each type onto the circuit board to implement each function separately.
Hence, there is a need for a PROM device which provides the capability of defining and programming the architecture of each output on an individual basis.